March 2016. Setting up the 200r4 transmission TV Dent cable for a Street Demon 1904 carburetor.
Correcting the geometry for correct line pressure, gear changes and smooth changes.
Very important -
Before explaining how to set this all up correctly I need to explain the dangers of not setting up correctly.
1. If the TV Dent cable is not adjusted correctly you can burn out an 200r4 transmission in a few miles!
2. If set up badly but not dangerously, the line pressures will be incorrect at each spead, the gears will
change at the wrong times and the changes will be sudden and not smooth.
Common mistakes are made for the following reasons -
1. You must not assumed Street Demon have put the holes for the dent cable in the correct place for your transmission.
They are not have done! Indeed different transmission companies need the holes to be in different places.
2. You must not assume, if the Lokar bracket is used, that Lokar may have included parts to adjust the geometry for
various makes of 200r4 transmissions. At the present time they do not.
It is therefore very important parts are adapted so that the geometry is corrected to make everything work perfectly.
This page (with many pictures) will explain why this is important and how to adapt the parts to have the correct geometry.
If you are using a different carburetor or a different bracket, these note will still help you.
The transmission used in this example is built by "Exteme Automatics".
The problem -
If you have any carburetor and a 200r4 transmission, hooking the TV dent cable onto the carburator,
then just pulling the cable tight before fitting the end stop ... if you then expect it to work correctly,
well ... it probably will not. Indeed this method will probably result in the destruction of your transmission!
The reason is, with just a reasonably tight cable, the piston in the transmission which determines the line (oil) pressure
will be extended by about 1 inch. At 1 inch of extension the line pressure at idle will be zero. Therefore you will be
pulling away from stationary with no transmission oil pressure and this will cause damage.
It will be damaged because -
1. The process is much more complicated than this and the piston needs to be further in
2. The cable adjustors on the bracket have no effect on the geometry and so cannot solve the problem
3. Don't assume the cable hole in the carburator is on the correct place no matter what the manufacturer claims.
Check with your transmission supplier -
You need to check the tech sheet from your transmission supplier to find out the piston movement required for your transmission
to be used safely (without being damaged) and for the gears to change smoothly and at the correct time. The transmission in this
example was built by "Extreme Automatics" and the piston movement must be 0.84 inches + or - 0.02 inches. That is between
0.82 inches and 0.86 inches. This movement is from fully in at full throttle (TWO) and fully out at idle.
Setting up -
To set up the TV Dent cable (sometimes incorrectly called the kick down cable as it does more than this in a 200r4)
the oil needs draining from the transmission, the sump pan must be removed and the piston must be observed. This really
needs 3 people. One person lying under the car or truck to observe the piston (just jack up the vehicle up including stands
for safety so you have clearance to lie underneath). One person is adjusting the cable on the carburetor, (a small stool to
stand on helps as the front of the vehicle ends up being a bit high). One person inside the vehicle pressing the throttle
pedal when told to. The person underneath needs a vernier caliper to measure the distance the piston has come out.
Piston movement -
It is always important to understand the theory if a problem is to be solved. It really helps if you know what you are doing!
When the accelerator pedal is pressed fully that is called "Throttle Wide Open" or "TWO", the piston in the transmission
must be fully pulled in. That means the amount of piston sticking out is zero. This is done by depressing the pedal fully and
at the same time pulling the cable, usually with some pliers and then locking it in position on the carburator with the
cable end stop. It will then probably slacken very slightly. If the person on the pedal releases it then presses it again,
the observed amount of piston might be about 1mm and it needs to be zero. The process could be repeated again making
sure it does not relax at all but it is easier to adjust with the cable adjusters on the bracket. They will pull the cable and
the piston will pull back in to the zero amount showing position. The left hand picture below shows the TWO piston fully in
with zero amount of piston showing. The right hand picture below shows the piston showing about 0.5 inches (not enough).
Once the TWO adjustment with the piston fully in is set and the end stop on the cable secure, let the pedal come up to the
idle position and measure the amount the piston is sticking out. (Everything from now on refers to the Extreme Automatics
200r4 transmission. For other transmissions, check what it needs to be). It needs to come out by 0.84 inches but it will be
fine if it is between 0.82 inches to 0.86 inches. It is unlikely it will come out the correct amount (as shown above right).
With the holes provided on the Street Demon carburetor it comes out about 0.5 inches to 0.6 inches depending on which hole
you use to hook on the cable. If you assumed this was correct and used the vehicle with this setting, you would not damage
the transmission as the piston is not further out than 0.86 inches. There will be plenty of line pressure at 0.6 inches,
however the gears will not change at the correct time and the shifts will be sudden. The pictures below show stage 1 on the
left in the TWO posion, piston fully in. On the right, stage 2 measuring the idle position of 0.85 inches after the mod below.
The geometry problem -
Sorry if it gets a bit complicated here but the problem is that the hole provided to hook the cable onto on the Street
Demon carburetor is too close to the point of rotation. When the throttle moves from idle to TWO, the mechanism on the
side of the carburetor turns about a point of rotation (a shaft) and the distance from that turning point to
the cable hole is not enough.
Think of it like this - A door has a hinge and you want to open the door through a fixed angle of 45 degrees. The distance
the door moves in a curved path if you are 1/2 way from the hinge to the edge of the door is less than the distance the
edge of the door moves (through the same angle). Or, putting it another way, if you push the door right at the edge, your
hand moves further than if you push it at a point 1/2 it's width from the hinge (through the same angle).
Getting back to the piston movement ...
The only way to pull more cable through and to move the piston a larger distance for the same amount of angle turned
through is to have a hole further away from the point of rotation.
The sketch below shows the cable moves further (for the same angle turned through) if attached at point C rather than
point A. You can see that C to D is a longer distance than A to B. To achieve this longer distance, the cable must be
attached further away from the point of rotation. C is further from the point of rotation (P) than A is.
Solving the problem -
With the Street Demon carburetor, a small plate with 3 holes in it needed to be designed and made. Two holes bolted to
two existing holes in the carburetor and the third hole was used to hook the cable on. This third hole was further away
from the point of rotation. Since fitting the adapter plate on the front of the mechanism gave clearance problems with
the rocker cover, (the upper two prototypes below were for the front) it was found it needed to bolt to the back of
the mechanism. Shown below left are earlier (front) designs made of aluminium. The final prototype made of mild steel
(painted black) and the final finished design made of stainless steel. Shown below right is the final design in
stainless steel with the Lokar "nail head bolt" through the bottom hole.
The two upper holes are 13mm apart and 6mm diameter. Two 5mm bolts with Nyloc nuts on the back were used to fix the
plate to the left and middle carburetor holes. The lower hole is 1/4 inch (a 6mm hole with a small round file to open
it up is fine if you don't have a 1/4 drill). The 1/4 inch lower hole is 19mm from the top left hole and 19mm from the
top right hole. The 1/4 inch diameter nail head bolt ended up being very close to the metal work of the carburator metal
work. Indeed it is necessary to rotate the nail head bolt so one of the flats is at the ideal angle to clear the metal
work. Once this plate was bolted to the carburetor, the cable was adjusted as detailed above. The piston movement from
zero sticking out at TWO to idle was 0.85 inches. This was within the limits of 0.82 to 0.86 inches. Shown below left are
the 3 original carburetor holes to attach the cable to. Street Demon say the centre hole is for a 200r4 transmission
and the left hand hole is for a Turbo 350 transmission. Our new plate must be attached to these two holes and the
new 1/4 inch hole is lower. The nail head bolt goes through the new hole and the cable attaches at this point.
This is shown in the right hand picture. This mod corrects the geometry for the Extreme Automatics transmission.
Refilling with oil -
Once everything has been fully checked, the sump pan can be put back on the transmission and the transmission re-filled
with oil. Please note when filling with oil, it may look like it is full when the dip stick shows full, however, the oil level on the
dip stick then falls when the engine is running. Indeed without putting in more oil, the gears would not engage. Top up to
the full mark on the dipstick with the engine running. In addition, the level needs checking again after driving the vehicle
as again the level may fall a small amount as the transmission is used and heats up. The dip stick should read full after
the transmission has done some work.
Oil pressure meter / gauges -
It is recommended an oil pressure gauge is used to check the oil pressure are correct after the TV Dent cable is set up.
However, whilst this is a good idea if you have a gauge, as long as the process above is followed correctly the oil
pressure will be correct. By all means check the pressures is you wish.
Electrical oil pressure in-cab gauges -
There are not many companies make an oil pressure gauge calibrated in psi. One available gauge is the VDO, 2 and 1/6th inch
diameter pressure gauge This is about £35 from companies like Summit Racing
. An electrical pressure sensor is also needed and
this is about $50.00. The part numbers are 350110 and 360406 (0-400psi). With an in-cab gauge it is possible to check there
is line pressure at idle. Other pressures can also be checked from the pdf chart from "Extreme Automatics". It is important
to read this pdf 200r4 tech
and use it as an important resource before starting this work.